Traditionally, a two-stage optical amplifier serves as a piece of equipment of an optical line.
Usually, the two stages of amplification are two different amplifying units connected in sequence in a pre-manufactured assembly, where the first unit is considered the first stage of the assembly and called an optical pre-amplifier, while the second unit (an optical booster) which follows the first one is considered the second stage. Such a configuration is usually implemented as a unidirectional integral assembly and, in the frame of the present description, will be called a conventional two-stage unidirectional configuration.
Each of the amplifying units usually comprises a single stage optical amplifier such as EDFA (Erbium Doped Fiber Amplifier) characterized by particular optical features (gain, flatness, noise, output power, etc).
The conventional two-stage unidirectional configuration is advantageous in that it has a higher maximum output power than a single stage optical amplifier and allows using relatively inexpensive amplifying units which might have differing optical parameters. However, when connecting such amplifiers into the two-stage configuration, degradation may appear in the flatness of the gain over the band of optical wavelengths supported by the combined amplifier. That's why the two amplifiers for the two-stage configuration are usually selected to have gain characteristics which allow compensating one another. The gain flatness can be adjusted by an operator during the manufacturing process, when interconnecting between the two stages. In practice, when manufacturing the assembly, the optical parameters of the first stage and the second stage of the unidirectional configuration are matched to produce the best performance, using a control interconnection between the two stages, that control interconnection comprising a communication channel and a micro controller, both provided within the assembly and serving the two mentioned stages of the unidirectional assembly. During operation of the assembly, the control interconnection continues fulfilling its functions for matching parameters of the amplifiers and coordinates their operation.
The unidirectional configuration may include a network element connected between the two stages (in the mid-stage). Such, intermediate elements have been used for signal attenuation, dispersion compensation, etc.
One example of such a configuration is described in U.S. Pat. No. 6,215,584.
It should be mentioned, that the conventional two-stage or multi stage unidirectional assembly, even when allowing independent control of parameters of the stages by the control circuit, is arranged in such a way that if a fiber cut occurs before the first stage amplifier, it behaves as a single integral device i.e., all stages of the assembly are shut down by the assembly controller, since the whole assembly appears to be useless in the absence of the input optical signal. Such a feature contradicts to inserting OADM (Optical Add Drop Multiplexer) in the mid-stage of the above-described configuration. One should recall that OADM is intended for receiving a WDM (wavelength division multiplexed) multi-channel optical signal, dropping at least part of optical information channels, and/or adding information to at least one of vacant optical channels for further transmitting a newly composed multi-channel multiplexed optical signal. Therefore, OADM in the mid-stage seems to be useless, since the complete shutdown of the two-stage configuration in the case of a fiber cut is harmful not only to the information received at the 1st stage, but also to the information added by the OADM.
The unidirectional two-stage configuration could theoretically be built from two separate amplifiers, which do not form an integrated assembly and thus each of the stages works independently when the other stage fails for any reason. Such a configuration would easily allow inserting OADM or any other element in the mid stage. However, in that case, high quality amplifiers with flat optical characteristics would be required and the configuration would become too expensive.